Our invention relates to magnetic tape transports, particularly to those for use with a magnetic tape cassette wherein a length of magnetic tape travels between a pair of independently rotatable hubs or reels within a generally boxlike housing. More particularly, our invention concerns a tape speed and tension control system in a magnetic tape cassette apparatus well adapted for the reading and/or writing of data in digitized form. The magnetic tape speed and tension control system of our invention incorporates improvements of the tape transport described and claimed in U.S. Pat. No. 4,163,532 issued to Sakai and assigned to the assignee of our instant application.
The noted Sakai patent suggests a tape speed control servomechanism having a speed sensing roller in frictional engagement with the tape through a prescribed one of a series of apertures defined conventionally in the cassette housing. The servomechanism measures the actual traveling speed of the tape by the speed sensing roller and controls a pair of drive motors, coupled one to each hub of the tape cassette, for constant tape speed. The speed sensing roller engages the tape through that one of the apertures which is located closest to that one of the cassette hubs which operates as the takeup hub during tape travel in a predetermined forward direction. The magnetic head of the apparatus engages the tape through the central one of the apertures.
Sakai's tape speed control servomechanism is associated with a tape tension control circuit whereby a supply side one of the cassette hubs during tape travel in either direction is energized to a varying extent necessary for constant tape tension. The tape tension control circuit has been so designed as to realize the same tape tension at the speed sensing roller during tape travels in both forward and reverse directions. However, the hub to hub tape path defined by the prior art apparatus is dynamically asymmetrical because of the offset arrangement of the speed sensing roller in frictional engagement with the tape. Consequently, according to Sakai's tension control, the tape tension at the magnetic head, located at the midpoint of the tape path between the two cassette hubs, has been greater during reverse tape travel than during forward tape travel.
We must point out that such unequal tape tensions at the magnetic head during forward and reverse tape transportations present no problem at all at the conventional tape speed. The higher tape tension during reverse tape run has incurred no rapid wear of the magnetic head.
The current trend in the information processing art, however, is toward greater capacities of all forms of storage media. It has recently been proposed to increase the storage capacity of magnetic tape cassettes to six times as much as heretofore. This requires higher tape tensions for proper data transfer contact between tape and head. In that case, if Sakai's tension control system were used for higher tape tensions, the tape tension during reverse tape travel would become inordinately high, possibly resulting in the rapid wear and inconveniently shorter useful life of the magnetic head.